The present invention relates to a display device, and more particularly to a reflection type liquid crystal display.
The liquid crystal display element is thin and light, and is used widely in various applications including the display of portable type information terminal. The liquid crystal display element itself does not emit light, but is a light receiving element which makes display by changing the light transmission intensity, and hence it can be driven by an effective voltage of several volts. Therefore, having a reflector at the lower side of the liquid crystal display element, the reflection type display device for displaying by reflected light of external light can operate by an extremely small power consumption.
A conventional reflection type color liquid crystal display element comprises a liquid crystal cell having a color filter, and a pair of polarizing films disposed on both sides of this liquid crystal cell. The color filter is provided on one substrate of the liquid crystal cell, and the color filter is formed on this substrate, and a transparent electrode is formed further thereon. By applying a voltage to this liquid crystal cell, the orientation state of the liquid crystal molecules is changed, and by making use of orientation changes of liquid crystal molecules, the light transmissivity of each color filter is changed to present a color display.
The transmissivity of one polarizer is about 45% at maximum, and the transmissivity of polarized light parallel to the absorption axis of polarizing film at this time is about 0%, and the transmissivity of vertical polarized light is about 90%. Therefore, in the reflection type liquid crystal display element using two polarizers, the light is emitted by passing the polarizing film a total of four times. Accordingly, by ignoring the absorption of color filter, the reflectivity is
(0.9)4xc3x9750%=32.8%
and, even in a monochromatic panel, the reflectivity is about 33% at maximum.
To realize a bright display, it has been proposed to use only one polarizing film disposed at the upper side of the liquid crystal cell, and enclose the liquid crystal cell by one polarizing film and a reflector (for example, Japanese Laid-open Patent No. 7-146469, and Japanese Laid-open Patent No. 7-84252). In these prior arts, since the light passes through the polarizing film only twice, by ignoring the absorption of the color filter, the reflectivity is
(0.9)2xc3x9750%=40.5%
and, at maximum, the reflectivity is improved by about 23.5% as compared with the constitution using two polarizing films.
In other prior art disclosed in Japanese Laid-open Patent No. 6-308481, without using color filter, the reflection type color liquid crystal display device for color display is realized by the birefringence of nematic liquid crystal layer of twist orientation of liquid crystal cell and the polarizing film.
Further, Japanese Laid-open Patent No. 6-175125 and Japanese Laid-open Patent No. 6-301006 disclose color liquid crystal display devices for making use of birefringence of liquid crystal layer and phase difference film.
These conventional liquid crystal display devices, however, had the following problems.
In the reflection type liquid crystal display element using two polarizing films, in the case of color display by using a color filter in this element, enough reflectivity for obtaining a sufficient brightness cannot be maintained.
In the reflection type liquid crystal display element using one polarizing film, color display is realized by using a color filter in this element, and the reflectivity is heightened to obtain a sufficient brightness. In this conventional constitution, however, black and white achromatic display is difficult, and in particular the reflectivity is low and black display is difficult.
In the case of the reflection type color liquid crystal display device for color display realized by the birefringence of nematic liquid crystal layer of twist orientation of liquid crystal cell and the polarizing film without using color filter, and the color liquid crystal display device for making use of birefringence of liquid crystal layer and phase difference film, enough reflectivity for obtaining a practical brightness can be maintained if two polarizing films are used, but since the color filter is not composed and color display is realized by using coloring by birefringence, multigradation multicolor display such as 16-gradation 4096-color display or 64-gradation full-color display is difficult in principle, and the color purity and color reproduction range is narrow.
In the reflection type liquid crystal display element of monochromatic display mode, a white reflectivity of white cannot be obtained even in the constitution using two polarizing films.
It is hence an object of the invention to present a reflection type liquid crystal display element bright in white display, high in contrast, and realizing a black and white achromatic display.
A reflection type liquid crystal display device of the invention comprises:
a liquid crystal cell having a first substrate, a second substrate, and a liquid crystal disposed between the first substrate and the second substrate,
a light reflecting member disposed at the second substrate side,
a polarizing film disposed outside of the first substrate, and
an optical retardation film disposed between the polarizing film and the liquid crystal cell,
in which the optical retardation film has a retardation value RC, and
there is the relation of formula 1 or formula 2 supposing the retardation value of the liquid crystal layer when an effective voltage Von is applied to the liquid crystal cell to be Ron.
xe2x80x83Ron+RC=(xcex/4)+(xcex/2)xc3x97(m)xe2x80x83xe2x80x83Formula 1
Ron+RC=(xcex/2)xc3x97(m+1)xe2x80x83xe2x80x83Formula 2
where (xcex) is the wavelength of light, and (m) is a positive integer including 0.
In this constitution, the white display is bright, and a high contrast is obtained. Moreover, a reflection type liquid crystal display device realizing a black and white achromatic display is obtained.